Parasites of animals and humans pose a worldwide problem. For example, schistosomiasis, after malaria, is the most common cause of human morbidity and mortality. Approximately 600 million people are at risk for schistosome flatworm infection, and approximately 200 million people in 74 countries are infected. Twenty million people (mostly children) have a severe form of the disease, and 200,000 die annually from the disease.
Mammalian parasites, such as platyhelminths of the genus Fasciola or Schistosoma and protozoans of the genus Trichomonas, can avoid immune elimination and survive for months or years in the fully immunocompetent vertebrate host. The surface of these parasites elicits a T cell-independent immune response characterized by the predominant production of IgM antibodies but fails to induce a T cell-independent response characterized by the production of IgG, IgE, and IgA isotype antibodies.
The production of IgG (as well as IgE and IgA) and their binding to the exterior of a pathogen is generally required for antibody-dependent cell-mediated cytotoxicity, a mechanism demonstrated to be effective in destroying parasitic worms. The binding of thymus-dependent antibodies (IgG, IgE, and IgA) to the exterior of extracellular pathogens is also generally required for phagocytosis by host macrophages and other immune functions included in a process of immune activation called “opsonization”. Opsonization is an immune mechanism frequently associated with destruction of extracellular protozoan parasites. It is generally believed that several mammalian parasites evade immune elimination by failing to induce surface-specific T cell-dependent functions, such as IgG, IgE, and IgA production.